Typical speech recognizers require at the input thereof an utterance detector 11 to indicate where to start and to stop the recognition of the incoming speech stream. See FIG. 1. Most utterance detectors use signal energy as the basic speech indicator.
In applications such as hands-free speech recognition in a car driven on a highway, the signal-to-noise ratio is typically around 0 dB. That means that the energy of the noise is about the same as that of the signal. Obviously, while speech energy gives good results for clean to moderately noisy speech, it is not adequate for reliable detection under such a noisy condition.